Method for operating an eye tracking device for multi-user eye tracking and eye tracking device

ABSTRACT

The invention relates to a method for operating an eye tracking device ( 10 ) for multi-user eye tracking, wherein images ( 24 ) of a predefined capturing area ( 14 ) of the eye tracking device ( 10 ) are captured by means of an imaging device ( 12 ) of the eye tracking device ( 10 ) and the captured images ( 24 ) are processed by means of a processing unit ( 16 ) of the eye tracking device ( 10 ). If a first user ( 26   a ) and a second user ( 26   b ) are present in the predefined capturing area ( 14 ) of the eye tracking device ( 10 ), a first information relating to the first user ( 26   a ) and a second information relating to the second user ( 26   b ) are determined on the basis of the captured images ( 24 ) by processing the images ( 24 ). Furthermore the images ( 24 ) are captured successively in a predeterminable time sequence.

The invention relates to a method for operating an eye tracking devicefor multi-user eye tracking, wherein images of a predefined capturingarea of the eye tracking device are captured by means of an imagingdevice of the eye tracking device and the captured images are processedby means of a processing unit of the eye tracking device. If a firstuser and a second user are present in the predefined capturing area ofthe eye tracking device, a first information relating to the first userand a second information relating to the second user are determined onthe basis of the captured images by processing the images. The inventionalso relates to an eye tracking device for multi-user eye tracking.

Most systems known from the prior art dealing with multi-user eyetracking usually are only able to track one user after the other but notat the same time. Furthermore, T. Villgrattner, H. Ulbrich: “Design andcontrol of compact high dynamic camera orientation systems” MechatronicsIEEE/ASME, Transactions on, on pages: 221 to 231, Volume: 16, Issue: 2,April 2011, ISSN: 1083-4435, doi 10.1109/TMECH. 2009. 2039223 disclosescameras, which can rotate and swivel to follow the eyes of the user,called active camera orientation system. Therein eye tracking onmultiple people at the same time is also described. For performingmulti-user eye tracking multiple cameras are used, especially one scenecamera to obtain an overview of the users present in the scene and twoactive camera orientation systems, one for tracking each user. Thisapproach has the advantage of getting a detailed image of each user, buteven several disadvantages. Each camera actually is responsible fortracking a single individual. Only the scene cameras sees multipleusers, but no eye tracking can be performed based on a scene cameraimage, partially due to the inferior image quality. This also means tobe able to track N users it will be necessary to have N active cameraorientation systems. Tracking multiple users with a single active cameraorientation system seems difficult due to the frame rate of 60 framesper second. To achieve an eye tracking rate of 30 frames per second inthe case of two users the system would have to take an image, reorientthe camera to the next person, take an image and move back. All of thishas to be done without a loss in frame rate due to the reorientationpart of the process. So for performing multiuser eye tracking a multicamera system is necessary comprising expansive pan-tilt cameras.

Therefore it is an object of the present invention to provide a methodfor operating an eye tracking device for multi-user eye tracking and aneye tracking device for multi-user eye tracking by means of whichmulti-user eye tracking can be performed in a simpler way.

This object is solved by various methods for operating an eye trackingdevice and various eye tracking devices disclosed herein .

According to the invention a method for operating an eye tracking devicefor multi-user eye tracking is provided wherein images of a predefinedcapturing area of the eye tracking device are captured by means of animaging device of the eye tracking device, the captured images areprocessed by means of a processing unit of the eye tracking device and,if a first user and a second user are present in the predefinedcapturing area of the eye tracking device, a first information relatingto the first user and a second information relating to the second userare determined on the basis of the captured images by processing theseimages. Furthermore, the images are captured successively in apredeterminable time sequence.

Capturing the images successively in a predeterminable time sequence,namely the images on the basis of which the information relating todifferent users are determined, makes it possible to use for exampleonly one camera to track multiple users at the same time. For thispurpose for example a camera can be used, which has a wide opening angleto be able to see multiple people, the imaging device also can compriseseveral cameras or image sensors and each image is a composition of thesingle image parts captured by each of the cameras or sensors to providea large capturing area. Moreover, the predefined capturing area can alsobe variable and/or adjustable in a predefined manner. But the mostimportant advantage of the invention is, that cameras of the eyetracking device are not assigned to respective users. For example oneand the same image can be used to derive information, i.e. the first andsecond information, relating to different users at the same time.

The first and second information preferably relate to the presence ofthe first and second user respectively and/or the gaze direction and orpoint of regard of the first and second user respectively. So on thebasis of the captured images user detection can be performed, that meansthat a presence of users can be determined, e. g. on the basis ofdifferent criteria, as well as eye tracking can be performed fordifferent users on the basis of the captured images.

Furthermore, according to an advantageous embodiment of the inventionthe first information and the second information are determined on thebasis of the same image of the captured images and/or on the basis ofdifferent and successively captured images. So for example each of thecaptured images can be used to derive the information relating to thefirst and second user at the same time or the information relating tothe first and second user can be derived from different images, whichare then captured consecutively. Also, some of the captured images canbe used for deriving both, the first and second information, and some ofthe images can be used for only deriving the first information and someof the images can be used for only deriving the second information. Sothere are advantageously many possibilities of using the consecutivelycaptured images, which facilitate adaption to the different situationsin many different cases.

For example the processing unit can process at least some of thecaptured images for determining whether the first and/or second user ispresent in the predefined capturing area. In general, this inventiondoes not only work with the first and the second user but also with anarbitrary number of different users. Also, if the eye tracking devicealready tracks the gaze of a first user on the basis of the capturedimages, the eye tracking device still checks at least in some of thecaptured images whether another user is present.

So it is a very advantageous embodiment of the invention that, if thefirst user is present in the predefined capturing area, the processingunit tracks the gaze direction and/or point of regard of the first useron the basis of at least some of the captured images and processes atleast some of the captured images for determining whether the seconduser is present in the predefined capturing area. Also here, the imagesused for tracking the already detected first user and the images usedfor detecting the presence of another user, like the second user, can bethe same or different ones, but if they are different ones, they havebeen captured consecutively.

So, advantageously, the processing unit can process each of the capturedimages for determining whether the second user is present in thepredefined capturing area or the processing unit can process only someof the captured images, in particular in predefined, especially regulartime intervals and/or with a frequency that varies in dependency on atleast one predefined parameter, which can be an external or internalparameter, like the average user number in recent history.

So for example, if in the beginning of this method no user is present inthe predefined capturing area, the eye tracking device can startcapturing images and processing these images for detecting users.Therefore, the processing unit can search in the images, whether facesor eyes or pairs of eyes are present in each of the captured images. Iffor example the first user is detected in one of these images, the eyetracking device can start an eye tracking process for the first user,meaning that the images captured in the following are used for trackingthe eyes of the detected user. Still the processing unit can processsome of these images, or even all of them at the same time for detectingfurther users. E.g. the processing unit can process the captured imagessuch that eye properties of the first user are derived from the capturedimages and the gaze direction and/or point of regard is calculated onthe basis of these eye properties, wherein at the same time every fifthof the captured images is used for checking whether new users arepresent in the capturing area. This checking can also be performed onevery captured image, in predefined time intervals or in an alternatingfashion. For searching for new users in an image and for determining eyeproperties on the basis of an image, it is advantageous to processimages differently. If a new user is searched in an image, e.g. thewhole image can be scanned for determining, whether there is a new faceor a new pair of eyes or the like. If a user has already been detectedand eye tracking is performed on the basis of the captured images, theapproximate position of the eyes of the found user in a newly capturedimage is already known as the eyes' position usually does not vary muchfrom image to image. So if an image is captured for eye trackingpurpose, the processing unit starts to search for the eyes of the userat a position, in which the eyes have been detected in the previousimage and usually only a small portion of the captured image has to besearched until the new position of the eyes is found again. So for thepurpose of detecting new users it may be advantageous to scan the wholeimage, whereas for tracking an already detected user, scanning to wholeimage is not necessary and only part of the image may be analyzed.Another alternative for searching for new users is instead searching inthe entire image at specific intervals to search in a small part of theimage in every frame and change the part that is being searched atperiodic intervals, adapting the different properties mentioned aboveaccordingly. This has the advantage that not the whole image has to besearched. So to better adapt to the requirements of searching users andtracking users respectively, it is a very advantageous embodiment of theinvention to define different states of the eye tracking device and toprocess images according to these states.

So, preferably, each of the captured images is assigned to at least oneof the following predefined states:

-   -   a user search state;    -   an eye tracking state; and    -   an eye tracking and user search state;        wherein the images are processed in dependency on the assigned        predefined state, especially wherein if one of the images is        assigned to the user search state, the processing unit checks        whether the first and/or second user and/or any further user is        present in one of the images, and if one of the images is        assigned to the eye tracking state, the processing unit        determines the gaze direction and/or point of regard of the        first and/or second and/or any further user based on eye        features detected in that one image, and if one of the images is        assigned to the eye tracking and user search state, the        processing unit determines a gaze direction and/or point of        regard of the first user based on eye features detected in that        one image and the processing unit checks whether the second user        and/or any further user is present in that one of the images at        the same time. Also additional states can be defined, like a        default state, in which none of the previously described steps        is performed.

So if an image, or generally a frame, which is the step of capturing andprocessing an image, is assigned to a user search state, the imagecaptured in that frame is checked for new users. If an image is assignedto the eye tracking state, eye features of an already detected user arederived from this image, and if an image is assigned to the combinedstate, namely the eye tracking and user search state, this image is usedfor deriving the eye features relating to an already detected user, andis also used for searching for new users. Depending on the predefinedstate, an image is assigned to, these images are processed differently.If for example an image is assigned to the user search state, the wholeimage may be roughly scanned for detecting new users. If an image isprocessed in the eye tracking state, only part of the image may beanalyzed, e. g. starting at a position, at which the eyes of the alreadydetected user were found in the previous image, and in the eye trackingand user search state, both analysis procedures of processing the imageare combined.

So, if at the beginning no user has been detected, the eye trackingdevice can be in the user search state until the first user is found.After the first user has been detected, the eye tracking device caneither alternate in predefined manner between the eye tracking state andthe user search state, or can alternate in predefined manner between theeye tracking state and the combined eye tracking and user search state,or can also be permanently in the eye tracking and user search state. Sofor example, if the first user has been detected, the eye trackingdevice can skip to the eye tracking state for tracking the eyes of thefound user and for example every third image switches to the user searchstate or the combined eye tracking and user search state, where theimage is either only used for searching for new users or additionallyused for searching for new users. If after some time the detected firstuser leaves the capturing area, the eye tracking device can switch againto the user search state until again a new user is found. This procedurecan be performed advantageously for any number of users. If for examplethree users have been detected, the eye tracking device can determinethe gaze direction and/or point of regard of each of the users at thesame time, especially on the basis of the same image, e. g. in the eyetracking state, or in the combined eye tracking and user search state,and again it can be searched periodically for new users.

Being permanently in the eye tracking and user search state if at leastone user has already been detected has the great advantage that newusers can be found very quickly, as every image is used for eye trackingof the found users as well as for searching for new users. On the otherhand, searching for new users on the basis of, e.g. every second imageor every fifth image has the advantage that the computation time forprocessing time processing the images can be shortened. These manypossibilities can be adapted advantageously to different situations. Iffor example usually the eye tracking device is used by two users andalready two users have been detected, the eye tracking device can skiponly from time to time to the user search state or the combined state,because it is unlikely that further users will join. If the eye trackingdevice is used by multiple and frequently changing persons, it may bepreferred to be permanently in the eye tracking and user search state(if already at least one user had been detected) to be able to detectfurther users very quickly.

According to another very advantageous embodiment of the invention theprocessing unit controls at least one component of the eye trackingdevice in dependency on the current predefined state. This is veryadvantageous, because this way the settings of the eye tracking deviceand its components can be optimized for searching users or trackingusers or both. For example images captured for searching for users canbe captured with a lower resolution, whereas images captured fortracking the eyes of an already detected user can be captured withhigher resolution. So for each predefined state of the eye trackingdevice specific settings of the components of the eye tracking devicecan be chosen.

Thereby, the at least one component may be the imaging device and/or anillumination unit for illuminating at least part of the predefinedcapturing area. This way advantageously the image capturing can beadapted to the respective state in which the images are captured andalso the illumination conditions can be optimized for each state.

In this regard it is a very advantageous embodiment of the inventionwhen the processing unit controls the illumination unit such thatillumination properties of the illumination unit, especially anillumination area and/or an illumination intensity and/or anillumination spectrum and/or the kind of light source used forillumination is adjusted in dependency of the current predefined state.For example, for images assigned to the eye tracking state theillumination unit can be controlled such that light emitted by theillumination unit is focussed on a predefined region of interest,especially which is only a subsection of the predefined capturing areaof the imaging device. This region of interest can be predefined on thebasis of the expected position of the eyes of a user. This expectedposition can be determined on the basis of the position the eyes of theuser in the previous image(s). The same applies if multiple users havebeen detected by the eye tracking device, so when the eye trackingdevice is in the eye tracking state for tracking the eyes of severalusers, the illumination unit can be controlled such that for each user aregion of interest, which is predefined on the basis of the expectedposition of the eyes of each user, respectively, is illuminated. On theother hand, for images, which are assigned to the user search state, theillumination unit is controlled such that a region is illuminated, thatis bigger than the above-named region of interest, and preferablyincludes the whole capturing area of the imaging device. As in the usersearch state captured images may be scanned as a whole for detecting newusers, it is very advantageous that the whole capturing area, of whichthe images are captured, is illuminated. Also, in the eye tracking statethe illumination unit can be controlled such that the illuminationintensity is greater than for example for images captured in the usersearch state. So, advantageously, the eye tracking accuracy can beenhanced by providing a good illumination of the region of interest inthe eye tracking state and at the same time in the user search statepower can be saved by reducing the illumination intensity.

Also respective settings can be chosen for the combined user search andeye tracking state, for example, the illumination unit can be controlledsuch that for capturing images in this combined state the wholecapturing area is illuminated by the illumination unit, and additionallythe predefined regions of interested are illuminated with higherintensity. Similar advantageous adjustments and adaptions can beachieved by controlling the imaging device in dependency of the currentstate of the eye tracking device. For example the processing unit cancontrol the imaging device such that imaging properties of the imagingdevice, especially the predefined capturing area and/or a gain of theimaging device and/or a focus value of the imaging device is adjusted independency on the current predefined state. For example for capturingimages in the eye tracking state the imaging device can be controlledsuch that the gain is greater than for images assigned to the usersearch state. Therefore better results can be achieved for eye tracking.Also in the eye tracking state the imaging device may be controlled suchthat a higher focus value is set, so that the imaging device focuses onthe region of interest for one of more already detected users and forthe user search state, the focus can be adjusted such that thecapturable area is maximum. Therefore, also the predefined capturingarea of the eye tracking device, or especially of the imaging device,can be adjusted in dependency on the current state of the eye trackingdevice.

According to another advantageous embodiment of the invention, if thefirst and/or second user is detected in one of the captured images, theprocessing unit derives user specific data from that image and comparesthe derived data with data of at least one stored user profile, and ifthe derived data match the data of the at least one stored user profile,the first and/or second user is determined to be an authorized user.This automatic user identification is especially advantageous withregard to multi-user eye tracking. On the one hand, this makes itpossible that only authorized users may use the eye tracking device andon the other hand user-specific data stored in the user profile can beautomatically derived from the storage medium and used for eye trackingof the identified user. For eye tracking usually meta information aboutthe currently tracked user is used. This meta information can forexample be derived from a calibration procedure at the beginning of asession. Such meta information may contain geometric properties aboutthe user, especially for identifying the user later on and also hiseyes, and especially the offset between the visual axis and opticalaxis. This offset varies from person to person and needs to be known forcorrectly calculating the gaze direction. So advantageously after a useris identified on the basis of a profile eye tracking data like theoffset can be used for tracking the eyes of this identified user andcalculate his gaze direction. The meta information also can compriseinformation about whether a user is wearing glasses or not. So for thispurpose the eye tracking device can comprise a storage medium where theuser profile and the corresponding meta information for each of theusers can be stored. If then the eye tracking device detects a user inthe captured images, it can first check whether some matching conditionwith regard to the stored profiles is fulfilled and if yes, the eyetracking device can start tracking the eyes of the user by using themeta information stored for this user in his profile.

Alternatively or additionally, for identifying users, usercharacteristics cannot only be captured on the basis of the capturedimages but also by other means like by means of a microphone capturingthe voice characteristic of a user and comparing it to a voice profile.So, in general, for identifying a user one or more characteristics ofthe user can be captured, like a face characteristic and/or a bodycharacteristic, especially at least one geometric property of the atleast one user and/or an eye characteristic, especially a geometricproperty of the at least one eye and/or a part of the at least one eye,and/or an eye motion pattern and/or whether the at least one user iswearing glasses and/or a refractive power of the glasses or a voicecharacteristic.

Moreover, by means of such automatic user identification, by means ofwhich authorized users can be determined, at the same time unauthorizedpersons can be detected as well. If the eye tracking device detects anew person in an image and then checks whether the detected person is anunauthorized person according to a predefined criterion, like there isno match with a stored user profile, then the eye tracking device canoutput a signal, like a warning signal, especially an optical and/oracoustical and/or haptic signal, wherein the signal can for example beoutputted on a display device or another component of the eye trackingdevice or a loudspeaker. The signal can also cause disabling the displaydevice of the eye tracking device of the eye tracking device or shuttingoff the display device or changing the content shown on the displaydevice. Therefore multi-user eye tracking can be performed in a verysafe way.

According to another advantageous embodiment of the invention, the eyetracking device comprises a display device, wherein the processing unitcontrols the display device in dependency of the first and secondinformation such that at least one first screen content is shown on thedisplay device in dependency on the first information and the secondscreen content is simultaneously shown on the display device independency on the second information. This allows in a very advantageousway different users to use a common screen or display and to control thescreen content, for example in dependency on the respective gazedirection. So the screen content can for example be a mouse cursor,especially wherein for each detected user a respective cursor is shownon the display device and each user can use and control his own cursorby means of his gaze direction. Also different screen sharing and screenmultiplexing techniques can be used to provide screen content for eachuser respectively. For example in a first area of the screen surroundinga first determined gaze point of a first user, a first screen contentcan be shown and in a second area surrounding the second gaze point of asecond user, a second and different screen content can be shown. Thefirst and second screen content can also be provided by differentapplications. Therefore, advantageously, the eye tracking device formultiuser eye tracking can be used also for providing applications on adisplay device, which can be used and controlled by each of the trackeduser separately.

The invention further relates to an eye tracking device for multiusereye tracking, wherein the eye tracking device comprises an imagingdevice configured to capture images of a predefined capturing area ofthe eye tracking device, a processing unit configured to process thecaptured images, wherein the processing unit is configured by processingthe images, if a first user and a second user are present in thepredefined capturing area of the eye tracking device, to determine afirst information relating to the first user and a second informationrelating to the second user on the basis of the captured images. Theprocessing unit is further configured to control the imaging device suchthat the images are captured successively in a predeterminable timesequence.

The preferred embodiments and advantages thereof described with regardto the method for operating an eye tracking device according to theinvention correspondingly apply to the eye tracking device according tothe invention, wherein in particular the embodiments of the methodconstitute further preferred embodiments of the eye tracking deviceaccording to the invention.

Moreover, the eye tracking device according to the invention can beconfigured as a remote eye tracker, configured to measure the positionand/or orientation of a user's eyes. The eye tracking device can becombined for example with a computer and delivers in real time theposition of the user's eye in front of a monitor, the gaze direction,and/or the position of the viewed object, usually a point of regard on amonitor.

Further features of the invention and advantages thereof derive from theclaims, the figures, and the description of the figures. All featuresand feature combinations previously mentioned in the description as wellas the features and feature combinations mentioned further along in thedescription of the figures and/or shown solely in the figures are notonly usable in the combination indicated in each place but also indifferent combinations or on their own. The invention is now explainedin more detail with reference to individual preferred embodiments andwith reference to the attached drawings.

These show in:

FIG. 1 a schematic illustration of an eye tracking device for amultiuser eye tracking according to an embodiment of the invention;

FIG. 2 a schematic illustration of an image captured by the eye trackingdevice according to an embodiment of the invention;

FIG. 3 a schematic illustration of a method for operating an eyetracking device for multiuser eye tracking according to an embodiment ofthe invention;

FIG. 4 a schematic illustration of a user search state of the eyetracking device according to an embodiment of the invention;

FIG. 5 a schematic illustration of an eye tracking state of the eyetracking device according to an embodiment of the invention;

FIG. 6 a schematic illustration of a combined state of the eye trackingdevice according to an embodiment of the invention;

FIG. 7 a flow chart illustrating a method for operating an eye trackingdevice according to an embodiment of the invention;

FIG. 8 a flow chart illustrating a method for operating an eye trackingdevice according to another embodiment of the invention; and

FIG. 9 a schematic illustration of a screen of an eye tracking devicefor providing multiuser applications according to an embodiment of theinvention.

Eye tracking, which allows multiple users to be tracked at the sametime, must pay attention to several special situations. For single usereye trackers, a common technique is to separate detection and trackingof eyes. Once a user is found, the device switches to a tracking mode,which tracks the found user, his face or his eyes over time. In order tosave computing time and transmission bandwidth, this is often combinedby capturing images only in the vicinity of the already found user, e.g. by using a hardware, region of interest of the camera, or by usingpan-tilt zoom cameras facing them to the tracked user. This conceptneeds to be adapted for multiuser eye tracking.

First of all, an eye tracking device and its components is describedwith regard to FIG. 1. FIG. 1 shows a schematic illustration of an eyetracking device 10 for multiuser eye tracking according to an embodimentof the invention. This eye tracking device 10 is configured as a remoteeye tracker, which comprises an imaging device 12, e. g. a camera or animage sensor, having a predefined capturing area 14 and a processingunit 16. The imaging device 12 captures images of the predefinedcapturing area 14, which are then processed by the processing unit 16.The eye tracking device 10 may further comprise an illumination unitwith optional light sources 18 for illuminating at least part of thecapturing area 14. Additionally, the eye tracking device 10 can comprisea storage medium 20 and a display device 22.

The concept of multiuser eye tracking, explained in more detail below,works for any kind of remote eye tracker, independently of thetechnique, whether it is based on infrared light or visible light,whether it has additional light sources and tracks corneal reflections,or not. Preferably, the eye tracking device 10 comprises an imagingdevice 12 with a camera having a wide opening angle to provide a largecapturing area 14 to be able to see multiple people. For example a wideangle camera can be used as imaging device 12, or also the imagingdevice 12 can comprise several cameras, the capturing areas of which arecomposed to one capturing area 14 of the imaging device 12.

So, by means of such an imaging device 12 images can be captured of thecapturing area, in which several users may be present. This isillustrated in FIG. 2, which shows an image 24 captured by the imagingdevice 12, in which exemplarily two users 26 a, 26 b are visible. Byprocessing captured images 24, the processing unit 16 detects visiblefaces and/or eyes in the image 24. If a new face, user and/or pair ofeyes is found, the device starts to track that user, especially inconsecutive images and the local change of the detected user and hiseyes is computed.

The main problem of a multiuser eye tracker is the handling of specialissues, which only occur when the device may track several users atonce. These special issues are the detection of new users as well as theidentification of users, which is explained later. With regard todetecting new users, the usual distinction in eye trackers betweendetecting a user and tracking his eyes must be handled differently. Itis possible (and even likely) that one user starts using the eyetracking device and another joins later. The eye tracking device musttherefore be able to detect new users, even when it is already trackinganother user. Therefore, conventional techniques, which e. g. use theregion of interest mode of the camera and only capture a small region ofinterest around the user's eyes or techniques using a pan-tilt zoomcamera and zoom to the eyes of a user would not work for multiple userswithout adaption.

According to an embodiment of the invention, when the eye trackingdevice 10 is already tracking a first user, the eye tracking device 10at the same time scans the captured images 24, especially the wholeimages for new users, which may enter the scene, e. g. enter thecapturing area. Each newly detected user starts a new tracking processso that multiple users may be tracked at the same time. This isillustrated in FIG. 3.

FIG. 3 shows a schematic illustration of a method for operating an eyetracking device 10 according to an embodiment of the invention. Thismethod can be logically separated in a detection thread for detectingnew users, shown on the left side of FIG. 3, and a tracking thread fortracking detected users, shown on the right side of FIG. 3. This methodstarts in step S10, in which the processing unit 16 waits for the nextframe, namely the next image 24, which shall be analyzed with regard tonew users. If such an image is received from the imaging device 12, theprocessing unit 16 checks whether new users are present in this image 24in step S12. If in step S14 no new users were found, these steps arerepeated again beginning in step S10. If in step S14 one or more newusers were found, a new tracking thread is opened for each new user instep S16 and the detection thread is repeated starting again from stepS10.

The tracking thread, which is started for each new user, runs parallelto the detection thread and comprises the following steps: The trackingthread starts in step S20, where again the processing unit 16 waits forthe next frame, namely the next image 24 captured by the imaging device12, on the basis of which one or more detected users shall be tracked.After receiving such an image 24 the processing unit 16 tracks the userin step S22, which means the eye positions in the image 24 are foundsimilar to existing eye tracking techniques. Next, in step S24 it ischecked whether the user was successfully tracked. If yes, properties ofthe eyes, like position, eye orientation and a gaze direction and apoint of regard are computed and this procedure starts again in stepS20. If the tracking was not successful in step S24, for example if theuser has left the capturing area 14, the tracking thread is terminated.So for each detected user, especially detected by the describeddetection thread, a new tracking thread is opened so that multiple userscan be detected and tracked at the same time. The eye tracking device 10finally provides one set of data per frame containing eye position and agaze direction and a point of regard for each user, which is currentlytracked. The user detection and user tracking can be performed on thebasis of the same captured images, but also on the basis of differentand consecutively captured images, which will be explained in moredetail in the following.

As the requirements for image processing may vary depending on whetheran image shall be used for detecting new users or tracking found usersor both it is very advantageous to define different states of the eyetracking device 10 and to assign each captured image 24 to acorresponding state and to process the images in dependency on theassigned state of the eye tracking device 10. These different states areexplained now in the following with regard to FIG. 4, FIG. 5, and FIG.6.

FIG. 4 illustrates a user search state USS. In this state an image 24 iscaptured in step S30 a and processed by the processing unit 16 in stepS32 a. When processing an image assigned to the user search state USS,the processing unit scans the whole image 24 for users, e. g. checkswhether faces, eyes, pairs of eyes are present in that image 24. Thisimage analysis is schematically illustrated on the left side of FIG. 4.The shown points in the illustrated image 24 shall illustrate that theprocessing unit 16 roughly scans the whole image, therefore the wholecapturing area 14, for new users.

FIG. 5 shows a schematic illustration of an eye tracking state ETS ofthe eye tracking device 10. The image 24 captured in step S30 b in theeye tracking state ETS is then also processed by the processing unit 16in step S32 b, however not for searching new users, but for trackingalready found users, and e. g. determine there respective gaze directionof each of the users. For determining a gaze direction or point ofregard or other properties of the eye a captured image 24 has to beanalyzed with regard to specific eye features, like a position of theeyes in the images 24, the pupil position, the position of glints and soon. The processing of images 24 in the eye tracking state ETS isillustrated on the left side of FIG. 5. In the eye tracking state ETSnot necessarily the whole image 24 has to be analyzed, but the analysiscan be restricted to defined regions of interest. Here exemplarily afirst region of interest ROI1 and a second region of interest ROI2 areschematically shown in the image 24. These regions of interest ROI1,ROI2 are determined on the basis of expected eye positions of alreadydetected users. If, like in this case, two different users have alreadybeen detected, for example in the user search state USS, the crudeposition of the eyes of the respective user can be estimated frompreviously captured images 24. So when processing images 24 in the eyetracking state ETS, the processing unit 16 can start searching the eyesof the respective users beginning in the defined respective regions ofinterest ROI1, ROI2, as it is very likely to find the eyes of therespective users in these regions of the image 24, so that computingtime can be saved. If the respective eyes of each of the detected userscan be found in these regions of interest ROI1, ROI2 eye properties orthe gaze direction can be determined for each of the users withouthaving to analyze other parts of the captured image 24.

Both of these states can also be combined, which is illustrated in FIG.6. FIG. 6 shows a schematic illustration of a combined state CS, whichis a user search and eye tracking state. In this state CS an image 24 iscaptured in step S30 c and processed by the processing unit 16 in stepS32 c, wherein by processing the captured image 24 already detectedusers can be tracked and at the same time the image 24 can be searchedfor new users. This is again schematically illustrated on the left handside of FIG. 6. In this case the processing unit 16 starts searching forthe eyes of the already detected users in the respective regions ofinterest ROI1, ROI2 and additionally analyzes the remaining image partsfor new users. Another alternative is instead of searching for users inthe entire image 24 is to search in a small part of the image 24 inevery frame and change the part that is being searched at periodicintervals. So the whole capturing area 14 can be scanned over time, e.g.over a predefined number of successively captured images 24.

The eye tracking device 10 can advantageously switch between thesestates to adapt to different situations. Furthermore, it is not onlyadvantageous to process images 24 in dependency of these predefinedstates, but optionally also to control other components of the eyetracking device 10 in dependency of the current state of the eyetracking device 10. Such components can be for example the illuminationunit with the light sources 18 and/or the imaging device 12. For examplein the user search state USS the illumination unit can be controlledsuch that the whole capturing area 14 is illuminated, wherein in the eyetracking state ETS the illumination can be focussed on the alreadydetected users to better illuminate the corresponding regions ofinterest ROI1, ROI2. Also for the combined state it is very advantageousto provide more light intensity in the regions of interest ROI1, ROI2than for the remaining capturing area 14. In a similar way also theimaging device 12 can be adjusted in dependency of the current state ofthe eye tracking device 10. So for example in the eye tracking state thegain, focus, or capturing area 14 can be concentrated on the respectiveregions of interest ROI1, ROI2 to achieve better eye tracking results.Moreover the following table shows some exemplarily settings for theuser search state USS, also called detection state and the eye trackingstate ETS:

Detection State USS Tracking State ETS Global Illumination LocalizedIllumination Large Region of Interest Localized Region of Interest Gainfor Optimal Detection Gain for Optimal Tracking Overview Camera ZoomCamera All Illumination sources Specific Illumination Sources

For the last two examples a multi camera and a multi illumination systemcan be used.

In the following some embodiments will be presented, in which thedifferent states of the eye tracking device 10 are combined in a veryadvantageous way. FIG. 7 shows a flow chart for illustrating a methodfor operating an eye tracking device 10 according to a first embodimentof the invention. This method starts in step S40, which is the usersearch state USS, explained with regard to FIG. 4. In step S42 it ischecked whether at least one user is present in the image 24 capturedand processed in the user search state USS. If no user has been found,the method goes back to step S40 and the eye tracking device 10 stays inthe user search state USS. If at least one user has been found in stepS42, the eye tracking device 10 switches to the combined state CS instep S44, in which a captured image 24 is used for tracking alreadyfound users as well as for searching for new users. In step S46 it ischecked whether at least one user is present in the last captured image24. If yes, the method returns to step S44 and the eye tracking device10 stays in the combined state CS. As long as users are present in thecaptured images 24, eye features, gaze directions and so on of thedetected users are determined on the basis of the captured images 24 andadditionally each image 24 is checked as to whether new users arepresent. If new users are found in the captured images 24 during the eyetracking device 10 being in the combined state CS, also properties, gazedirections, and so on of these new users are determined in this combinedstate CS. These steps are repeated as long as users are present in thecapturing area 14. If all of the users left this capturing area 14 andno user is detected in step 46 anymore, the eye tracking device 10switches back to the user search state USS in step S40 and the methodstarts all over again. This method has the great advantage that newusers can be detected very quickly, as in the combined state CS eachimage, which is used for tracking already found users, is also used forsearching for new users.

FIG. 8 shows another flow chart for illustrating a method for operatingan eye tracking device 10 according to another embodiment of theinvention. In the beginning of this method the eye tracking device 10again is in the user search state USS in step S50. As long as no usersare found on the basis of captured images 24, which is checked in stepS52, the eye tracking device 10 stays in the user search state USS.Otherwise, if at least one user is found on the basis of an image 24,the eye tracking device 10 switches to the eye tracking state in stepS54. In this eye tracking state ETS as already explained the found usersare tracked on the basis of captured images 24. During the processing ofeach image 24 in the eye tracking state ETS it is checked in a step S56if still at least one user is present. If not, the eye tracking device10 switches back to the user search state USS in step S50. If still atleast one user can be tracked, it is further checked in step S58, if apredefined number of images has already been captured in the eyetracking state ETS. Here in FIG. 8 i denotes the number of the imagecaptured in the eye tracking state ETS and n and N denote integers,wherein N is a fixed integer, like 5, and n can be an arbitrary integer.So if N for example is 5, the eye tracking device 10 switches for everyfifth captured image to either the combined state CS or the user searchstate USS in step S60. In this state an image is captured for searchingfor new users, either only, like in the user search state USS, oradditionally to tracking found users, as in the combined state CS. Afterthat it is checked in step S62 again whether at least one user is stillpresent, if not, the method again begins in step S50, and, if yes, theeye tracking device 10 again switches to the eye tracking state ETS instep S54. If in step S60 new users were found also these new users aretracked in the subsequent eye tracking state ETS in step S54. Soaccording to this embodiment, if at least one user is present in thecaptured images, this user is tracked. Furthermore, if new users aredetected, also these new users will be tracked. However, the userdetection is not performed for each of the captured images, but onlyafter a predefined number of captured images, like every second image,every third image and so on. Thereby, advantageously, computing time canbe saved.

Another aspect of the invention, which shall be explained in thefollowing, is the user identification. Most eye trackers use metainformation about the currently tracked user. Typically, a calibrationprocedure at the beginning of a session is used to collect suchinformation. It may contain geometric properties about the user, hiseyes, the offset between the visual axis and optical axis, whetherhe/she is wearing glasses etc. While this information can be treated asconstant meta data for single user eye trackers, the situation isdifferent for a multiuser eye tracking where users may leave andre-enter the scene all the time. Therefore it is a very advantageousembodiment of the invention that the tracking device 10 is configured toidentify new users in order to use user specific data. For this purposefirst of all a calibration procedure can be performed, which is a veryimportant aspect of achieving high accuracy in eye tracking. Every useris different, though, so each individual needs to perform its owncalibration sequence to obtain a different set of correction parameters.For this purpose, a calibration procedure can be run for each user andthus gaze correction data can be obtained for each user respectively.Thereby, also for each user characteristics for identifying a respectiveuser later on can be captured. These sets of user identification dataand gaze correction data can be stored in the storage medium 20 of theeye tracking device 10 in a corresponding user profile. Also calibrationprocedure can be performed by means of the eye tracking device 10 on twoor more persons at the same time.

These stored profiles can now be used advantageously to identify usersagain. When multiple people are sitting in front of the same eyetracking device 10, it will be advantageous to match calibrationinformation with the correct individual. Incorrectly matching user andgaze correction data will cause a large error in the point of regard ofthe individual. Therefore, it is very advantageous, when the processingunit 16 is configured to identify new detected users on the basis ofuser characteristics, which may be derived from the captured images andto match these characteristics with a corresponding user profile and usethe eye tracking data stored in this user profile for eye tracking ofthis identified user.

According to another embodiment of the invention, in a multiuser system,like the multiuser eye tracking device 10, users might have differentrights. Some examples for this are:

-   -   In a development environment where person A helps person B. In        this scenario person A should only have the right to point,        whereas person B should still have control of the system.    -   In a household situation in front of a TV only one person should        have the right to change the channel, similar to their being        only a single remote control. If this is not the case, this        could cause frustration and incorrect selections of channels.    -   Some individuals might have permissions on some parts of the        screen, whereas on other parts they are not allowed to perform        any changes, depending on the currently active windows.

Therefore it is very advantageous when, according to another embodimentof the invention, such permission data, rights, or priority data foreach user of the eye tracking device are stored additionally in thecorresponding user profile. If a user is detected by the eye trackingdevice 10 and identified on the basis of his profile, the permissiondata can be derived and the eye tracking device can decide which tasksthe user is allowed to perform e.g. by means of gaze control accordingto his/her permissions.

Furthermore, it may be useful to know if a third party is looking at ascreen or not. This can be in security-related situations, where it isimportant to not allow other people to see what is being shown on ascreen, like the display 22 of the eye-tracking device 10. On the basisof the stored user profiles unauthorized persons can also be detectedand identified as such by means of the eye tracking device and differentactions can be performed to ensure security. For example a warning canbe provided and outputted by the eye tracking device 10 to the mainuser, the screen 22 can be disabled temporarily, some fake data ordifferent screen content can be shown on the display 22, some kind ofinverted foveated rendering can be performed, in which in the areaaround the gaze point of the unauthorized person a default image isshown instead of the actual content. This will only affect the usage ofthe display 22 of the main user in situations where both the main userand the unauthorized person look at the same point.

Furthermore, some embodiments of the invention will be described whichrelate to very advantageous multiuser applications. There is a widevariety of situations, environments and places, in which multiple peoplelook at the same screen, like the display 22 of the eye tracking device10. Here a few sample applications, for which multiuser eye tracking infront of a single device, especially a single display 22, is useful andcan greatly enhance the experience of the user or simplify thefulfilment of tasks.

For example the calculated point of regard of a first user can be shownon the display 22 as a pointing device to create interest for a seconduser. Often the situation occurs that the person a wants to drawperson's B attention to a specific part of the screen 22. Instead ofperson A pointing with a finger or a mouse cursor, according to thisembodiment of the invention, it is possible to use the respective pointof regard as a method of growing attention to a location or event.

According to another embodiment foveated rendering can be used to reducethe amount of computation and energy being used by a screen 22 bylimiting the area of the screen, on which data is displayed to thoseparts where the user is actually looking. The area close to thefocussing area of the user will here be called core. Areas further awaywill be called peripheral. There are multiple ways that the core and theperipheral area can be treated depending on the usage scenario. Forexample, in a 3D rendering situation the core can be rendered with highquality, whereas peripheral areas are rendered with lower quality. Insome scenarios it will even be possible to show some default content inperipheral areas, e. g. a black image to save power for illumination.The border between the core and peripheral areas can also be dealt within many different ways, e. g. using medium quality rendering, blurringor even a hard edge. With multiuser eye tracking foveated renderingcould be done for multiple individuals on the same screen 22.

This is schematically illustrated in FIG. 9. Here a situation is shown,where two users of the eye tracking device 10 are looking at the displayscreen 22. The eye tracking device 10 determines the point of regard ofeach user on the screen 22. The respective points of regard are heredenoted by P1 and P2. These points of regard P1, P2 are shown in FIG. 9only for illustrative purpose and are not really presented on thedisplay screen 22. Furthermore, screen content C1, C2 is only shown in apredefined area A1, A2 surrounding the respective points of regard P1,P2. By this advantageous embodiment two or more people can share thesame screen 22, and even for each of them different screen content canbe shown, for example one is reading a book and the other is watching avideo. So the different screen content C1, C2 referring to therespective user, can also be provided by different applications runningon the eye tracking device 10 or the computer, the eye tracking device10 is coupled with. This works in a similar way as foveated rendering,except that each person has its own virtual desktop, and on the screen22 it is being displayed in a foveal radius around a given person'sdetermined point of regard P1, P2 a portion of the virtual desktop whichmatches that user. As long as latency is low and the points of regardP1, P2 of both users do not simultaneously overlap, the users will havethe illusion of each of them looking at an own screen 22. Technicallythis can be achieved in many different ways, one option being the use ofone virtual screen for each user and a mask that decides, which virtualscreen content C1, C2 is displayed for each pixel. Between these virtualscreens a blurred border can be used to allow for a slightly less sharpedge between the two virtual screens.

To decide which virtual screen is shown on which pixel of the display 22there are a few alternative decisions that can be taken, especially whenmultiple users are looking into the same area of the screen 22. Forexample for each pixel it can be decided which point of regard P1, P2 iscloser and the virtual screen can be picked accordingly. Additionally, ablurred edge between the two virtual screens can be shown.Alternatively, an area around each point of regard P1, P2 is shown withthe respective virtual screen. Optionally, in a conflict area the imagescan be blurred with some transparency value.

Further use cases for multiuser eye tracking are for example games, TVcontrol, telephone conferences or use in surgery rooms. With regard togames, in the past many games used to be played with multiple peoplesitting at the keyboard triggering actions in the game using differentkeys. Instead of two people using the same keyboard, it will be possibleto control their characters using gaze instead. The idea of games withmultiuser eye tracking can be particularly interesting for socialevenings in an assistive environment. This would allow playing softwareversions of board games such as chess and checkers, or monopoly and riskas two more complex examples by maintaining the social element of gamesevening.

In a TV scenario multiple people sit in front of the display 22. Eachindividual might want to change the currently active settings of the TV,such as the channel, volume etc. With a multiuser eye tracking system,like the eye tracking device 10 according to this invention it ispossible to realize a TV that enables each user to control all or asubset of the settings.

Also online call systems like skype can add a feature to show where aperson in a phone conference is looking. This can be used for creatinginterest, but is can also be used for other purposes, such as settingthe volume specifically on the person that has the attention of the userand reducing the volume for others.

With regard to the use in surgery rooms, in a surgery setting multipledoctors and assistants might require different information from ascreen. Using multi-gaze tracking any individual can set what is on ascreen or part of a screen.

So, according to the invention and its embodiment a method for operatingan eye tracking device for multiuser eye tracking and a multiuser eyetracking device are provided, which make it possible that multiple userscan be tracked at the same time in a very simple and easy way. This canbe used in a wide variety of applications, some of which are mentionedabove.

LIST OF REFERENCE SIGNS

-   10 eye tracking device-   12 imaging device-   14 capturing area-   16 processing unit-   18 light source-   20 storage medium-   22 display-   24 image-   26 a, 26 b user-   ROI1, ROI2 region of interest-   P1, P2 point of regard-   C1, C2 screen content-   A1, A2 predefined screen area-   USS user search state;-   ETS eye tracking state;-   CS eye tracking and user search state;

The invention claimed is:
 1. A method comprising: capturing a first setof one or more images; determining a value of a user search parameterbased on one or more numbers of detected users in the first set of oneor more images; capturing a second set of a plurality of images;selecting a subset of the second set of the plurality of images with afrequency based on the value of the user search parameter; searching, ineach of the subset of the second set of the plurality of images, for arepresentation of an undetected user; and performing, for each of thesecond set of the plurality of captured images, eye tracking upon arepresentation of a detected user, wherein performing eye tracking uponthe representation of the detected user is performed using a portion ofthe image and searching for a representation of an undetected user isperformed on the entire image.
 2. The method of claim 1, wherein theuser search parameter is based on a number of currently detected usersin the first set of one or more images.
 3. The method of claim 2,wherein the subset includes fewer of the second set of the plurality ofimages when the number of currently detected users is greater.
 4. Themethod of claim 1, wherein the user search parameter is based on anaverage number of detected users in the first set of one or more images.5. The method of claim 4, wherein the subset includes fewer of thesecond set of the plurality of images when the average number ofdetected users in the first set of one or more images is less.
 6. Themethod of claim 1, wherein the user search parameter is based on avariation in the number of detected users in the first set of one ormore images.
 7. The method of claim 6, wherein the subset includes fewerof the second set of the plurality of images when the variation in thenumber of detected users in the first set of the one of more images isless.
 8. The method of claim 1, wherein the subset of second set of theplurality of images includes less than all of the second set of theplurality of images.
 9. The method of claim 1, further comprising:capturing a third set of a plurality of images; determining a secondvalue of the user search parameter; selecting a subset of the third setof the plurality of images with a second frequency based on the secondvalue of the user search parameter, wherein the second frequency isdifferent than the frequency; and searching, in each of the subset ofthe third set of the plurality of images, for a representation of anundetected user.
 10. The method of claim 1, wherein the numbers ofdetected users in the first set of one or more images includes at leastone of zero detected users, one detected user, or two detected users.11. An apparatus comprising: a camera configured to capture a first setof one or more images and a second set of a plurality of images; aprocessor configured to: determine a value of a user search parameterbased on one or more numbers of detected users in the first set of oneor more images; select a subset of the second set of the plurality ofimages with a frequency based on the value of the user search parameter;search, in each of the subset of the second set of the plurality ofimages, for a representation of an undetected user; and perform, foreach of the second set of the plurality of captured images, eye trackingupon a representation of a detected user, wherein perform eye trackingupon the representation of the detected user is performed using aportion of the image and search for a representation of an undetecteduser is performed on the entire image.
 12. The apparatus of claim 11,wherein the user search parameter is based on a number of currentlydetected users in the first set of one or more images.
 13. The apparatusof claim 11, wherein the camera is further configured to capture a thirdset of a plurality of images and wherein the processor is furtherconfigured to: determine a second value of the user search parameter;select a subset of the third set of the plurality of images with asecond frequency based on the second value of the user search parameter,wherein the second frequency is different than the frequency; andsearch, in each of the subset of the third set of the plurality ofimages, for a representation of an undetected user.
 14. A non-transitorycomputer-readable medium encoding instructions which, when executed,cause a processor to perform operations comprising: determining a valueof a user search parameter based on one or more numbers of detected userin a first set of one or more captured images; select, from a second setof a plurality of captured images, a subset of the second set of theplurality of captured images with a frequency based on the value of theuser search parameter; search, in each of the subset of the second setof the plurality of captured images, for a representation of anundetected user; and perform, for each of the second set of theplurality of captured images, eye tracking upon a representation of adetected user, wherein perform eye tracking upon the representation ofthe detected user is performed using a portion of the image and searchfor a representation of an undetected user is performed on the entireimage.